1. Field of the Invention
This invention relates to flightcraft charge control apparatii and more particularly to charge control apparatii that have electrically isolated second bodies that establish a potential reference for charge removal from the craft.
2. Prior Art
Static electricity has compromised manned flight from the days of hydrogen filled balloons to the present.
In those early days, the primary concern was that of spark ignition of the balloon gases. Later, when aircraft radio was introduced, communication by this means was compromised by static-electricity-produced interference. The ignition problem was addressed by substituting helium for hydrogen, and the communication problem by improved communication equipment, the matter of static electricity and its control surviving unresolved.
Static electricity now further compromises manned flight, particularly that of hovercraft, spacecraft, and stealthcraft.
For hovercraft, the primary concern is that of electrocution of ground personnel. Tests of CH-53E helicopters over sandy terrain confirm charging rates of 70 microamperes and potentials to ground of 140 kilovolts. Extrapolation suggests that first contact with the craft by ground crews can be contact with 400 kilovolts-
For spacecraft the primary concern is lighting. Spacecraft launch paths parallel cloud-to-ground lightning paths, and charged craft can initiate the stroke and change the lightning path to one that includes the craft.
For stealthcraft the primary concern is that of detection. Clear air testing of CH-53E helicopters confirms charging rates quite capable of producing radio noise. Stealthcraft flying under similar conditions can be expected to produce the same detectable signatures.
The helicopter test data was taken from (U.S.) Naval Research Laboratories Memorandum Report Number 5676, Electrostatic Charging of the CH-53E Helicopter, Pechacek et al. This report also makes the very important point that the key to flightcraft charge control is a charge reference uncompromised by the electrical isolation of flight.
Tranbarger et al attempt this in Helicopter Model Studies For On-Board Electrostatic Sensors prepared for the U.S. Army Research and Technology Laboratories by Southwest Research Institute, San Antonio, Tex. These studies resulted in an excellent bibliography but, unfortunately, no viable charge reference.
Buser, U.S. Pat. No. 3,874,616, and de la Cierva, U.S. Pat. No. 3,427,504 use as reference surface charge density at a particular point on a helicopter, and sample this with a field mill. While surface charge measurements at an arbitrary point on an electrically isolated conducting sphere suffice to accurately predict the potential of that sphere with respect to infinity, they are less than satisfactory for predicting the potential difference between a hovering helicopter and ground. Tranbarger recognizes this and attempts to refine prediction by measuring surface charge density at additional points, correcting for craft altitude, and adjusting for the charge density of the cloud surrounding the craft. While enough such measurements could conceivably lead to a satisfactory prediction of the potential of a bare helicopter with respect to a well defined ground plane, prediction is sorely compromised when, for example, the "ground plane" is a crew member attempting hook-up to the craft from atop a load, or when most of the charge expected to be sampled on the craft itself has been attracted to an external load by the ground plane.
Ayres, U.S. Pat. No. 2,386,084, and Andresen, U.S. Pat. No. 2,386,647, use external elements to sense the potential of the craft. Their arrangements are less than satisfactory, however, because both the craft and the "sensors" are exposed to the same charging environment, with the result that both can be elevated to the same undesirable potential without invoking any mechanism to remedy the condition.
While no sensors or references suitable for flightcraft charge control are known from the prior art, a multitude of arrangements for removing charge from a flying craft are known. Charge removal by corona discharge is, for example, practiced passively by Leake, U.S. Pat. No. 2,320,146, and actively by de la Cierva, U.S. Pat. No. 3,427,504, and Parkinson, U.S. Pat. No. 3,986,681. The Tranbarger study also proposes charge removal by this means, but, like this invention, with the corona electrodes disposed in the engine exhaust stream.
Ayres, U.S. Pat. No. 2,386,084, and Welsh, U.S. Pat. No. 3,283,210, use droplets to remove charge from the craft itself, as does one embodiment of the present invention, but do not suggest using droplets to maintain a reference body at the desired potential.
Other prior-art charge removal arrangements include charge capture (Andresen, U.S. Pat. No. 2,386,647) and grounding. Examples of the latter include Crossley, U.S. Pat. No. 1,757,111, Corbin, U.S. Pat. No. 3,893,005, and, of course, conventional aircraft, where charge is leaked to ground via tires, etc. upon landing.